1. Field of the Invention
The present invention relates to an optical system, and is suitable for an image pickup optical system used in an image pickup apparatus such as a silver halide film camera, a digital still camera, a digital video camera, a monitoring camera, or a broadcasting camera.
2. Description of the Related Art
Image pickup optical systems used in an image pickup apparatus, such as a digital still camera or a digital video camera, including a solid image pickup element are required to be small, have a wide view angle, etc. In addition, when the angle of incidence of a light ray that is incident upon the solid image pickup element is large, peripheral light quantity at a screen is reduced, as a result of which shading and color misregistration occur. Therefore, image pickup optical systems are required, for example, to have good telecentric property.
U.S. Pat. Nos. 7,440,196 and 7,940,478 each disclose an image pickup optical system in which an aperture stop is disposed closer to an object side than a central portion of the image pickup optical system, and the number of lenses of a lens unit that is disposed closer to the object side than the aperture stop is reduced, to reduce the size of an entire system.
An image pickup optical system in which a lens unit having a low positive refractive power is disposed in front of an aperture stop with the number of lenses being small and in which a lens unit having a relatively high positive refractive power is disposed behind the aperture stop easily achieves a wide view angle while ensuring back focus. However, with the aperture stop as a boundary, spherical aberration and asymmetric aberration (such as coma aberration) occur often when the asymmetry of a lens configuration at an object side and that at an image side are large.
In particular, if the F number of an image pickup optical system is reduced (that is, if the brightness is increased), spherical aberration occurs often, thereby making it difficult to achieve good optical performance.
An embodiment of the present invention provides an optical system which, while its entire system is small and while providing a wide view angle, can easily provide a high-quality image in the entire area of a screen, and can easily maintain good optical performance even during image stabilization.